1. Field of the Invention
The present invention relates to a DA converter and a solid-state imaging device. More particularly, the invention relates a current-controlled DA converter, and a solid-state imaging device equipped therewith.
2. Description of the Related Art
There is a current-controlled DA (Digital-Analog) converter. This current-controlled DA converter is also called a current-added DA converter (see, for example, JP-A-11-261420 (Patent Document 1)).
FIGS. 11A and 11B are simple circuit diagrams of current-controlled DA converters. FIG. 11A shows a power-supply-based DA converter 500, and FIG. 11B shows a ground-based DA converter 600. The DA converters 500 and 600 respectively have variable current sources 510 and 610 inside. The variable current source 510, 610 is controlled based on a control signal CNT to change the current that flows across a resistor R provided at the output part. This configuration allows a voltage to be applied to the resistor R to thereby generate a desired voltage level at an output terminal DACOUNT.
The variable current source 510 has a plurality of current source parts 520-1 to 520-n as shown in FIG. 12A. In the variable current source 510, the current flowing across the resistor R is controlled by controlling the current source part 520-1, . . . , 520-n based on a control signal CNT-1, . . . , CNT-n externally input. The variable current source 610 has a plurality of current source parts 620-1 to 620-n as shown in FIG. 12B. In the variable current source 610, the current flowing across the resistor R is controlled by controlling the current source part 620-1, . . . , 620-n based on the control signal CNT-1, . . . , CNT-n externally input.
The specific configuration and operation of the variable current source 510 in FIG. 12A will be described by way of example. Hereinafter, an arbitrary one in the current source parts 520-1 to 520-n is called current source part 520. Further, an arbitrary one in the control signals CNT-1 to CNT-n is called control signal CNT.
The current source part 520 includes a constant current source 521, a pair of PMOS transistors 522 and 523, and an inverter circuit 524. The PMOS transistor 522, 523 has a source connected to the constant current source 521. The PMOS transistor 522 has a drain connected to a power supply Vdd, and the PMOS transistor 523 has a drain connected to the output terminal DACOUNT. In addition, the inverter circuit 524 is provided between the gates of the PMOS transistors 522, 523 to carry out contradictive control in response to the control signal CNT.
This configuration of the current source part 520 switches the PMOS transistors to be turned on from one PMOS transistor to the other according to the state of the control signal CNT. That is, when the control signal CNT has an L level, the PMOS transistor 522 is turned on, and the PMOS transistor 523 is turned off. At this time, the constant current source 521 is not connected to the output terminal DACOUNT, so that the voltage at the output terminal DACOUNT is not influenced. When the control signal CNT has an H level, on the other hand, the PMOS transistor 522 is turned off, and the PMOS transistor 523 is turned on. At this time, the constant current source 521 is connected to the output terminal DACOUNT, so that the current according to the current value of the constant current source 521 flows across the resistor R, thus lowering the voltage level of the output terminal DACOUNT.
With the above-described configuration of the variable current source 510, the current flowing across the resistor R can be controlled by controlling the current source part 520-1, . . . , 520-n according to the control signal CNT-1, . . . , CNT-n. That is, the voltage level of the output terminal DACOUNT is lowered by increasing the number of those control signals CNT in the control signals CNT-1 to CNT-n which are to be set to an H level, and the voltage level of the output terminal DACOUNT is increased by decreasing the number of those control signals CNT which are to be set to an L level.
Such a DA converter 500, 600 can be used in, for example, a solid-state imaging device. In the solid-state imaging device, a reference voltage which changes stepwise is output from the DA converter, and compared with an analog signal read from each pixel in the pixel array section. The voltage value of the analog signal read from each pixel in the pixel array section is detected according to the comparison time, and is output as a digital value (see, for example, JP-A-2005-323331 (Patent Document 2)).